Lasers with a high output power are required for a number of applications, such as materials processing, particle acceleration, military applications, and laser induced fusion for energy production. Lasers for these applications are required to provide high energy, high repetition rate pulses. One of the challenges associated with obtaining stable and reliable pulse generation is managing the heat generated in optical elements of the laser. Heating may occur in a variety of components such as optical gain media, Pockels cells, Faraday isolators, frequency conversion stages where some optical absorption occurs and many other components in which absorbed energy is converted to heat. Conventional lasers producing high-energy pulses use rods with water cooling or slabs without active cooling as a gain medium. The pulse energy and/or the pulse repetition rate provided by such lasers is not high enough for laser induced fusion and other applications, such as laser-driven particle accelerators.
Thermal management in optical gain media arranged as slabs has been investigated under a US Department of Energy contract, the results of which have been published as “Thermal Management in Inertial Fusion Energy Slab Amplifiers”, Sutton and Albrecht, Lawrence Livermore National Laboratory, 1st International Conference on Lasers for Inertial Confinement Fusion, Monterey, Canada, May 30-Jun. 2 1995 and as Sutton, S. B. & Albrecht, G. F. (1995), “Thermal management in inertial fusion energy slab amplifiers”, Proceedings of SPIE 2633, 272-281. These papers describe the use of gas-cooling of large aperture slabs where the beam propagates through the cooling medium. The consequences of poor thermal management are thermally induced aberrations and thermally induced birefringence, both leading to a degradation of the quality of the transmitted beam. Thermally induced deformation or expansion of the gain material can cause beam steering. In the extreme case, the thermally induced stresses can lead to cracking of the gain medium. The arrangement described by Sutton and Albrecht uses an end-pumped configuration with the slabs of gain medium oriented normal to the pump laser beam. The pump laser beam was provided from semiconductor laser diodes. The end-pumped arrangement used gain media segmented into a series of thin slabs with cooling channels there between. A gas is pumped at high velocity through the channels to remove heat from the slabs. As mentioned above, the pump laser beam and emitted beam pass through the cooling medium. Turbulent flow and spatial variations in the cooling rate were previously considered to be major barriers to achieving good beam quality. Turbulent flow was considered to introduce non-uniform scattering losses, whereas spatial variation in cooling rate was considered likely to result in unacceptably large variations in optical path length in the system. The paper describes that the use of helium as a cooling medium can overcome these problems.
A later project, known as the Mercury Laser, is described in “Activation of the Mercury Laser: A diode-pumped solid-state laser driver for inertial fusion”, Bayramian et al., Advanced Solid-State Lasers 2001 Topical Meeting and Tabletop Exhibit, Seattle, Wash., Jan. 29-31, 2001. The project is also described in A. Bayramian et al. (2007), “The mercury project: A high average power, gas-cooled laser for inertial fusion energy development”, Fusion Science and Technology 52(3), 383-387. The project goal was to design a laser capable of producing 100J pulses having a pulse length of 2-10 ns and a repetition rate of 10 Hz. FIG. 1 is a schematic diagram showing the system for cooling the slabs of gain medium. The cooling system 5 comprises a heat exchanger 10, channels for routing the gas stream 30, a circulating fan 20, and laser amplifier 50 which includes vane mounts 60. The heat exchanger 10 cools the gas after it has passed by the gain media. The circulating fan 20 pumps the gas around the system towards the laser amplifier. The laser amplifier 20 includes slabs of gain medium 62 mounted in aerodynamic vanes 60. FIG. 2 shows the vanes 60 mounted in the amplifier. The vanes 60 are stacked such that the slabs of gain material lie adjacent to each other and coincident with windows 82 in the amplifier manifold. Around the edge of each slab is edge cladding 84 to locate and support the slabs in the vanes.
Small gaps 86 between the vanes, and between the vanes and the manifold, provide channels through which the cooling gas flows. Gas is pumped through the amplifier manifold. The gas first approaches the nozzle section 70 which conditions the gas stream by narrowing the cross section within the manifold to match the stack of vanes. The gas next passes through straight section 80 which has channels or gaps between the vanes 60, cooling the slabs. After the channel section 80 the vanes narrow in the diffuser section 90 and the gas merges back together.
The optical gain medium is pumped by a beam arranged normal to the plane of the slabs. The output beam generated is also normal to the slabs. The narrow channels between the vanes and the curved leading edge accelerate the gas to produce turbulent flow between the slabs. Turbulent flow provides better cooling than laminar flow. The diffuser section 90 decelerates the gas and the flows merge back together at the trailing edge of the vanes. The diffuser section 90 tapers in a series of steps. The pressure drop across the channels was found to be small enough to prevent the formation of wake disturbances. The measured wavefront output from the amplifier includes a wavefront distortion due to heating of the gas as it traverses the amplifier. The gas used in the cooling system was helium at a gas pressure of 4 bar within the channels of the straight section. The gas velocity within the channels is Mach 0.1 and the mass flow rate is ˜1 g/sec. The gas operates at around room temperature.
Operating the cooling system at lower temperatures and higher pressures than those used in Mercury brings about certain benefits.